AbstractIt is of great importance to enhance ability of charring formation for polystyrene (PS) to overcome non‐charring feature during burning. Here, we demonstrate a kind of ternary intumescent flame retardant composition composed by a novel ionic liquid butyltriphenylphosphonium‐chelated orthoborates (IL) with self‐expanded flame retardant, ammonium polyphosphate (APP) and expandable graphite (EG), to improve flame retardancy of PS. This ternary composition promotes limited oxygen index (LOI) value to 26.6 ± 0.5% and UL‐94 V0 level for PS composite with 15 wt% total loading. In addition, the peak value of heat release rate and smoke production rate of PS composite with ternary composition are separately reduced by 79.7% and 70.7% comparing to neat PS as well as 44.4% and 29.2% to its IL‐free binary counterpart in cone calorimeter test. Meanwhile, the ternary composition displays compact charring bulk constructed by large‐expansion‐volume vermiform expanded graphite. The TG‐FTIR technique is used to analyze gaseous profiles. The rheological property discloses that expansion behavior of the ternary composition contributes to robust viscoelasticity of composite and advantageous network construction at high temperature. Given the fire performance of the ternary composition, the combination of IFR and IL provides an opportunity for rational design and fabrication of functional flame‐retardant system.